Creating a pure spin current in graphene

Feb 07, 2011 By Miranda Marquit

(PhysOrg.com) -- Graphene is a material that has the potential for a number of future applications. Scientists are interested in using graphene for quantum computing and also as a replacement for electronics. However, in order to realize these graphene applications, a solid understanding of how spin current works in graphene is important.

One of the goals is to achieve pure spin current in graphene. Pure spin current is a spin current with zero charge current, which means electrons with different spins travel toward opposite directions, K S Chan tells PhysOrg.com via email. Chan is a professor at the City University of Hong Kong. Working with Zijing Lin, a professor at the University of Science and Technology of China in Hefei, and, Qingtian Zhang, a student of the CityU-USTC joint Ph.D. program, Chan studied adiabatic pumping in graphene as a way to generate spin current. Their work is published in Applied Physics Letters: Spin current generation by adiabatic pumping in monolayer graphene.

Spin current is an important tool of studying spins in graphene, Chan explains. With spin current, you can create polarization in a particular region, and you can study the behavior of the spin in that particular region. Chan points out that spin current is important in the development of a graphene quantum computer. Additionally, he points out that graphene is the material of choice for spintronics, which some hope will be able to replace electronics.

Spintronic devices are believed to be faster and consume less power than electronic devices, Chan continues. Understanding how spin works in graphene could be an important part of making a breakthrough in spintronics. Chan and his colleagues use a method called adiabatic quantum pumping to generate spin current for study.

Chan describes the technique: [Adiabatic pumping] is a quantum phenomenon in which a DC current is generated without a DC voltage. Two AC voltages are applied to the graphene and a DC charge current can be generated through adiabatic quantum pumping. Adiabatic means the rates of change of the voltages are very slow in comparison with the speed at which the electrons travel through the graphene structure.

On top of that, the team created asymmetry between electrons with different spin using the ferromagnetic proximity effect. A ferromagnetic thin film is deposited on graphene. Electrons with different spins under the ferromagnetic layer will have different energies and therefore respond differently to adiabatic pumping, Chan says. As a result of these different responses, pure spin current is generated, with different spins traveling in opposite directions. What is so special about the present method is that a pure spin current can be generated at some Fermi energy without an external magnetic field, which is important for making nanosized devices.

Fundamentally, the work done by Chan and his colleagues show that it is possible to generate pure spin current in graphene without magnetic field. This could lead to more practical applications in quantum computing and perhaps, later, spintronics. The next step, though, is to learn how spin current can be detected.

Spin current is difficult to detect, Chan explains. Its not like the charge current which can be easily measured by a voltmeter. He admits that there are other important issues that need to be studied regarding spin in graphene, but Chan points this out: To develop graphene spintronic devices, we need to know how to measure the spin current in graphene.

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Fundamentally, the work done by Chan and his colleagues show that it is possible to generate pure spin current in graphene ... The next step, though, is to learn how spin current can be detected.

Did anyone else fall out of their chair laughing when they read that paragraph. I must say I have a pretty hard time wrapping my head around spin currents and spin reservoirs and the like. This made it all better.

nuge: what is the proposed method by which spin can do work? Maybe I just don't get it. At least we can't measure it yet either.

I'm wondering if spin currents might one day be used for power transmission, if they result in less loss of power than charge currents.

I'd argue that you couldn't do that. A normal charge current applies "pressure" like water flowing through a hose, but a charge current is going to be generating a magnetic field where the power harvested will depend on the outside area of the wire that you engulf in the power receiving device.

Additionally, the rate of spread of the magnetic domain down the wire would be a serious issue that would determine the rate of power flow. You probably couldn't do an AC type of current since the pulses would bleed into and cancel each other out over long distances - think a heat distribution over time.

There are many more problems I am thinking of, but I'm too lazy to write any more. ;D

A spin current can be used in imaging. E.g. with magentic resonance imaging you create an imbalance in the number of spins that point in a certain direction within a region (say a human body). By then applying a magentic field you tilt those spins out of their plane of rotation. When they relax back to their original plane you get a signal.

In magentic resonance imaging the spin imabalnec is only about 1 in 6000 IIRC (which makes for a very low signal to noise ratio (SNR) )

If you can create a spin current in a material then you'd probably be able to image that material using MRI with a very nice SNR because you can get a much higher rate of spin imabalnce.

nuge: what is the proposed method by which spin can do work? Maybe I just don't get it. At least we can't measure it yet either.

Spin can maintain itself very long time without power input, which means computers use far less energy and memory becomes non-volatile.

Some advantages of this are computers with near-instant boot time, and hand held gadgets who's power supply lasts 10 times as long, because the only thing that uses a significant amount of power would be the screen and the audio.

Spintronic RAM should theoretically only use power when you are changing it's data value, whereas electronic RAM uses a certain amount of power just to hold an electric charge. Which is to say, even if a "bit" is doing no "work" at all, it costs electricity in an electronic computer.

In a spintronic RAM energy is consumed only when a variable's value changes, at least in theory.

Because so little energy will be used, you won't need cooling, further reducing energy consumption.

Spin can maintain itself very long time without power input, which means computers use far less energy and memory becomes non-volatile.

Spin is maintained indefinitely in elementary particles as it is quantized. It requires no power input whatsoever.

I'm wondering if spin currents might one day be used for power transmission,

The spin of one particle cannot be used to get power. You can use the gradient in spin pools to get power (as was shown in an article a few days ago on physorg on spin ensembles used to power a Maxwell daemon). This basically means that you would have a number of particles with one type of spin in one place and with another type of spin in another place. Using this low entropy state you can get to a high entropy state and grab some energy in the process.

I doubt that using spin pools is particular effective at creating an energy flow. The storage time of the 'energy' might be very long (until the spins flip through random fulctuations)

Huh? Entropy isn't an energy source. Moving from low to high entropy only does work when there is a force at play. "Brownian motion" doesn't generate work unless you use a heat exchanger, slowing the motion.

My understanding of spin is that it generates a magnetic moment. If this is the case, maybe a reservoir has some magnetism that could be used to do work by creating a spin current, but I haven't seen anything like this in any literature. I would think such an effect would make the spin current easy to detect, but that doesn't seem to be the case.

The example you gave from the previous article is a thought experiment and lacks the method of extracting the work from the reservoir.

Using quantum spin for imaging and information has been documented. Anybody have a clearer explanation or a source that details spin's potential use as a power source, outside of the Maxwell's daemon thought experiment to generate work from information?